1. Field of the Invention
The present invention relates to image-forming apparatuses such as copying machines, facsimile machines, printers and so on, more particularly, image-forming apparatuses that have electrophotographic systems.
2. Description of Related Art
Conventionally, high quality images with long-term stability have been desired from image-forming apparatuses that have electrophotographic systems. In response to this, particularly, improvements have been made in terms of reliability and print quality of digital-copying machines or laser printers which convert image information into digital signals and form an electrostatic latent image with light to form an image.
From the viewpoint of improving the print quality and reliability, it is especially important to establish high image quality, high stabilization of the image (especially high abrasion resistance) and high stabilization of the photoreceptor.
As the photoreceptors, those using photosensitive organic materials have been widely used in general due to cost, productivity, low pollution, and so on.
As the photoreceptors using organic materials, the following are known: a photoreceptor using photoconductive resins as represented by polyvinyl carbazole (PVK); a photoreceptor using a charge-transfer complex as represented by PVK-TNF (2,4,7-trinitrofluorenone); a photoreceptor of a pigment dispersion-type using a binder as represented by a phthalocyanine binder; a photoreceptor of a functional separation-type used in combination with a charge-generating material and a charge-transport material, and so on.
In the photoreceptors using a photosensitive organic material, since the photosensitive layer has a low-molecular-weight charge-conveyance material and an inert polymer as the main components, the organic photoreceptors are generally flexible.
If the organic photoreceptor is used repeatedly in the electrophotographic process, there is a tendency for abrasion to be caused by a mechanical load of developing and cleaning systems.
If abrasion of the photosensitive layer progresses, the following are prone to be accelerated: reduction in charge potential of the photoreceptor; degradation of light sensitivity; scumming due to scratches on the photoreceptor; decrease in image density; and image degradation.
Accordingly, there is a need to improve abrasion resistance in the organic photoreceptor.
In particular, it is difficult to improve the abrasion resistance of the organic photoreceptor because of downsizing thereof due to downsizing and speeding up of the electrophotographic device.
Therefore, it is important to improve abrasion resistance.
From this viewpoint, the use of photoreceptors of a functional separation-type has increased due to their superior photosensitivity and durability characteristics. In addition, it is able to molecularly design a charge-generating material and a charge transport material individually and so on by selecting functional separation-type.
In a method of forming an electrostatic latent image using the photoreceptors of the functional separation-type, the photoreceptor surface is charged and irradiated with light, and this light passes through a charge conveyance layer to be absorbed into a charge-transport material disposed inside of a charge-generation layer.
An electric charge is then generated in the charge-generation layer. This electric charge is applied to the charge-conveyance layer at a boundary surface between the charge-generation layer and the charge conveyance layer.
The electric charge then moves through the charge conveyance layer via an electric field, which neutralizes the photoreceptor surface.
As a result, the electrical potential of the photoreceptor decreases at the irradiated position. The non-irradiated part of the photoreceptor surface that still has an electrical potential (residual potential) is an electrostatic latent image.
As a method to improve abrasion resistance of the photoreceptor, a method of adding fillers into the outermost periphery of the photoreceptor is widely known.
Another method using a charge-transport material of a polymer-type instead of using that of a low-molecular-weight-type (CTM) is also widely known.
However, there has been a problem as follows in an image-forming device having the photoreceptor with an abrasion resistance in its outermost periphery.
As shown in FIG. 16, there is a problem that the rotation number fails to increase up until the electric potential on the photoreceptor surface reach a target potential range that is close to a target charge potential.
If an image-forming process has been started before the charge potential on the photoreceptor surface charged (charge potential) reaches the target charge potential range, which results in a lack of the image density of a toner image, a deviation in the image density during the toner image formation. These problems cause a remarkably uneven image density in the toner image.
Therefore, a pre-rotation of the photoreceptor is carried out as an image-forming apparatus, for example, in JP2009-145704A. Namely, as an image-forming is carried out after waiting for the charge potential of the photoreceptor reaches the target charge potential range in general, the pre-rotation of the photoreceptor is carried out by rotating the photoreceptor while charging it with a charging device prior to the image-forming process.
In the pre-rotation process of the photoreceptor, if the rotation number of the photoreceptor (pre-rotation number) is set to be an unnecessary large number, which results in a short life of the photoreceptor.
Therefore, it is desirable to set the pre-rotation number of the photoreceptor in minimum necessary. For this purpose, the image-forming apparatus in JP2009-145704A performs a pre-rotation process with a pre-rotation number selected as the following.
Firstly, a range of appropriate pre-rotation numbers is predetermined from preset pre-rotation numbers in accordance with the total rotation number of the photoreceptor.
Next, a total rotation number, usage environment, and a print number of times of the photoreceptor are measured by measuring devices for the photoreceptor.
Then, the pre-rotation number appropriate for the pre-rotation process is selected from the range in terms of the measured data and the pre-rotation process is carried out.
The image-forming apparatus in JP2009-145704A determines the pre-rotation number from the next view point.
This view point is aiming for the image-forming process starts at timing early from the time point that the charge potential of the photoreceptor reaches the target charge potential range.
However, the present inventors found out, as a result of extensive studies, an image degradation problem is caused by a residual electrical potential (electrical potential of exposure section) fluctuating and unstable even after the electrical potential of the photoreceptor has reached the target charge potential range in a process of exposing to form the latent image on the charged photoreceptor surface.
FIG. 17 is a graph illustrating a relationship between rotation number of the photoreceptor in the pre-rotation process and the residual electrical potential.
As shown in FIG. 16, the electrical potential of the photoreceptor approaches the target charge potential gradually by increasing the pre-rotation number in the pre-rotation process.
Eventually, the charge potential (absolute value) of the photoreceptor reaches a predetermined reference potential or more to be within the target charge potential range.
On the other hand, as shown FIG. 17, the more the pre-rotation number of the photoreceptor in the pre-rotation process increases, the more amount of change in the residual electrical potential reduces gradually.
However, the pre-rotation number necessary to stabilize the residual electrical potential is larger than that necessary to let the charge potential reach the target charge potential range.
Therefore, as the image-forming apparatus in JP2009-145704A above, if the pre-rotation number of the photoreceptor is set so as to start image-forming at timing earliest possible from the time point that the charge potential reaches the target charge potential range, an image is formed before the residual electrical potential is stabilized.
If the amount of change in the residual electrical potential is large, an amount of change in an electrical potential for developing becomes also large and a toner concentration fluctuation becomes remarkable.
Therefore, if the image-forming process has been started before the residual electrical potential is stabilized, this results in problems to cause an image failure.
This problem is a lack of the toner concentration in a toner image or a toner concentration deviation in the toner image due to toner concentration fluctuation in the image-forming process.
However, in some of the image-forming apparatuses, there is an image-forming apparatus to apply a voltage with a polarity opposite to the target charge potential onto a transfer member facing the photoreceptor in order to apply a transfer bias between the photoreceptor and the transfer member.
Then, the image-forming apparatus transfers a toner image on the photoreceptor onto a body to be transferred such as an intermediate transfer body or a recording medium etc. due to the transfer bias.
As a result of a study by the present inventors, in such image-forming apparatuses as above, if the pre-rotation process is carried out in a condition of the transfer bias being applied, as shown in FIG. 18, the residual electrical potential does not change. It was confirmed that the problem described above does not occur.
It relates to that a charge (here assuming this charge is negative charge) on the photoreceptor surface charged by a charging process is neutralized by the transfer bias.
More specifically, a positive charge is applied to the photoreceptor surface by a transfer current flowing by the transfer bias. Then, the negative charge on the photoreceptor surface which is charged by the charging process is neutralized by the positive charge.
Typically, an amount of the positive charge applied by the transfer bias is less than that of the negative charge by the charging process. Therefore, the negative charge remains on the photoreceptor surface even after the neutralization by the transfer bias.
Here, if a charge potential on the photoreceptor surface has already reached the target charge potential range, a charge potential on the photoreceptor surface in the next charging process reaches an electrical potential nearly equal to that in the previous time (the target charge potential).
Therefore, the amount of the negative charge remaining on the photoreceptor surface after the charging process in the second time is equal to that in the first time.
After that, the amount of positive charge applied by the transfer bias is equal to that in the previous time. Therefore, the amount of the negative charge remaining on the photoreceptor surface after neutralization by the transfer bias is also equal to that in the previous time.
In summary, if the charging process and applying the transfer bias are repeated, there is no stack of the charge on the photometer surface. Therefore, the residual electrical potential on the photoreceptor surface does not change.
On the other hand, a case will be explained that the pre-rotation process of the photoreceptor is carried out while irradiating and neutralizing the photoreceptor surface with neutralization light.
As well as a method of forming an electrostatic latent image, the positive charge generated by neutralization light moves to the photoreceptor surface due to a potential difference with the negative charge on the photoreceptor surface charged by a charging process.
Then, a positive electric field moved to the photoreceptor surface counteracts the negative charge on the photoreceptor surface and then the photoreceptor is neutralized.
In this case, the positive charge is trapped into the layer of the photoreceptor in the course of the movement of the positive charge. Then, the trapped positive charge is accumulated gradually and the residual electrical potential on the photoreceptor surface gradually increases.
An increased amount (the amount of change) of the residual electrical potential by this positive charge trap is decreasing with a progressive increase in the number of repeating the neutralization by the neutralization light and the charging process.
The residual electrical potential stabilizes before too long. However, it requires a considerable pre-rotation number of times until the residual electrical potential stabilizes as described above.
Moreover, in addition, in a case that a pre-rotation process is carried out while irradiating and neutralizing the photoreceptor surface with the neutralization light under a condition of applying the transfer bias, the residual electrical potential does not change and the problem described above does not occur.
This reason is as follows. Even if the neutralization with the light is carried out against the photoreceptor surface after applying the transfer bias to it, there is little negative charge on the photoreceptor surface due to a neutralization effect by the transfer bias, so that the positive charge generated in the photoreceptor layer does not move to the photoreceptor surface.